Recently, in active matrix type liquid crystal display devices (LCD) using thin film transistors (TFT), increase in size of the screen and high resolution power have proceeded rapidly. LCDs are generally classified into amorphous Si-TFT using amorphous silicon (a-Si) as an active layer and a polycrystal Si-TFT using polycrystal Si as an active layer, in which the former has been commercialized to products mainly as large-scaled television sets taking advantage of reduced cost and the latter has been commercialized to products mainly as medium-to-small sized displays taking advantage of high resolution and high speed operation. Particularly, in the polycrystal crystal Si-TFT of high mobility (carrier mobility, hereinafter simply referred to as mobility), investigation has been promoted on reduction in cost and improvement in reliability with an aim of application to organic electroluminescence (organic EL) display devices.
The method concerned with the active layer of the polycrystal Si-TFT includes a method of forming a polycrystal Si film in the process for film formation and a method of forming an amorphous Si film and then crystallizing the amorphous Si film by using lamp overheating or laser, thereby modifying the film to a polycrystal Si film. Since the film is formed on an inexpensive and less heat resistant glass substrate in either method, it requires a processing temperature of about 500° C. or lower. As the method of crystallizing the amorphous Si film, a crystallization method by laser annealing providing high throughput and less warping of the glass substrate is predominant. Further, flexible display devices having a polycrystal Si-TFT fabricated on a resin substrate or a resin film have also been developed vigorously in recent years.
The laser annealing method used for crystallization of the amorphous Si film includes those disclosed in WO97/22141 and WO97/22142.
The outline for the process of crystallization method by laser annealing disclosed in WO97/22142 is to be described below.    (1) An amorphous Si film with a thickness of about 60 nm to about 130 nm is formed by a plasma CVD method.    (2) The amorphous Si film is crystallized under the following conditions.    (2-1) The substrate temperature in laser irradiation is controlled to a temperature from about room temperature (25° C.) to about 400° C.    (2-2) The laser irradiation time is controlled within about 10 ms for an identical portion of the film (semiconductor film).    (2-3) The laser irradiation energy is controlled to 1000 mJ/cm2 or less.    (2-4) The laser oscillation source has no restriction and crystallization is conducted by using lasers such as excimer laser (ArF, XeCl, XeF, KrF), YAG laser, Ar laser, dye laser, and carbonic acid gas laser.    (3) After forming a gate insulator and a gate electrode, the gate electrode is fabricated.    (4) Impurities are implanted to source and drain regions by an ion implantation method at a temperature of 350° C. or lower.    (5) A interconnection layer is formed.